This invention relates to a process for treating dyed and undyed polyester materials with an ultraviolet absorber agent to overcome or inhibit the destructive effects of ultraviolet light. The invention is more particularly directed to a process for intimately combining an ultraviolet light absorber with a polyester material so as to alter the structure of the polyester to inhibit or overcome the destructive effects of ultraviolet light. A bath containing a non-volatile liquid polyhydric alcohol and an effective amount of an ultraviolet light absorber which is compatible with polyester material is dissolved in the bath, the temperature of the bath being maintained in a temperature range of from 250.degree. to 430.degree. F. The polyester material is then contacted with the bath for a relatively short period of time of up to 30 seconds so that an effective amount of the ultraviolet light absorber, together with the polyhydric alcohol, is substantially uniformly dispersed throughout the polyester material. The polyester material is subsequently washed to remove excess bath from the surfaces of the polyester material and is then subjected to a heat treatment step of up to 15 seconds to remove any remaining polyhydric alcohol dispersed or entrapped in the polyester material.
Polyester material which are formed as a reaction product of poly basic acids and polyhydric alcohols are not, per se, resistant to ultraviolet light (see Randall U.S. Pat. No. 2,920,978, column 1, lines 50 through 56). The reaction product of terephthalic acid and ethylene glycol forms a synthetic material of polyethylene terephthalate which is commercially produced under the brand name of Dacron as a fiber or Mylar as a film and which undergoes degradation on exposure to sunlight unless means are taken to render the polyester material stable to the destructive effects of ultraviolet light.
Changes in the weather and long exposure to sunlight, in which ultraviolet rays are present deteriorate and embrittle polyester material, generally within one year or less. To overcome this problem, it has been known to coat polyester material with an ultraviolet light absorber with or without an adhesive binder, but preferably with an adhesive binder to prevent weathering, loss and leaching loss of the ultraviolet light absorber.
These prior art methods have proved unsatisfactory since cracks develop in the surface of the coating and apertures and perforations extend through the coating to form areas accessible to ultraviolet light to cause deterioration of the polymer and render the product unusable. Further, the coating method is suitable only on a base at least 5 mils in thickness, the process being impractical for the thinner gauges which are the most important commercial embodiments. Furthermore, where coating is employed as a method for applying the ultraviolet light absorber, any crack or opening formed in the coated material will provide a point of access for ultraviolet light which will then cause the polyester material to rapidly disintegrate due to the deterioration of the polymer chain and thereby render the product unusable. Also, where an adhesive is employed to bond two plastic members together or a plastic member to another member, the adhesive requires protection from ultraviolet light because without it it will discolor and also may break down and separate from the polyester material.
It has been known to employ ultraviolet light absorber of the benzophenone class which is described in U.S. Pat. No. 2,920,978 of Randall. However, when practicing the process of Randall it has been found that the polyester material has only limited periods of stability. Thus, when the benzophenones of Randall are treated or applied to a polyester material such as Dacron and allowed to weather, the treated material will discolor and eventually deteriorate due to ultraviolet light attack. This has been borne out wherein Dacron cloth becomes cracked and brittle after about 900 hours of exposure in a fadeometer test.
It was accidentally discovered that minute traces of the polyhydric alcohol remained in the film or in the cloth of polyester materials, even after washing at room temperature and drying at temperatures of about 212.degree.F. Further, it had been found that becuase of the presence of the polyhydric alcohol in the treated polyester material certain processes could not be carried out, for example, the metal deposition on polyester film would not adhere thereto because of the presence of the polyhydric alcohol. Since the polyhydric alcohol will leach from the polyester material, it also carries with it small amounts of ultraviolet light absorber so that after a period of time when substantially all of the polyhydric alcohol has been removed from the film the film becomes brittle and will crack and deteriorate under attack by ultraviolet light. It has been found that by practicing the process of the invention herein treated dyed or undyed polyester materials with an ultraviolet light absorber intimately combined therewith will render the treated polyester material weatherable, stable and resistant to the destructive effects of ultraviolet light. The process comprises the steps of immersing the polyester material in an undyed or dyed condition in a bath containing a non-volatile liquid polyhydric alcohol and the bath containing an effective amount of an ultraviolet light absorber preferably of the benzophenone class or the benzotriazole class, the bath being maintained at a temperature of 250.degree. to 430.degree.F. so that when the polyester material is contacted with the treating bath liquid an effective amount of the polyhydric alcohol and ultraviolet light absorber will be intimately combined with the polyester material. Thereafter, the polyester material is washed to remove excess treating bath liquid from the surface of the polyester material and it is then subjected to a heat treatment step for a relatively short period of time, for example, 3 to 30 seconds at a temperature of from 350.degree. to 450.degree.F. to remove the excess polyhydric alcohol entrapped in the polyester material. Thus, by the removal of the excess polyhydric alcohol the treated polyester material is rendered ultraviolet light stable. It was unexpected that only the entrapped polyhydric alcohol would be removed from the treated polyester material and that the vaporized polyhydric alcohol did not carry any entrapped ultraviolet light absorber from that treated material during the heat treatment step.